Researchers in Carnegie Mellon University's School of Computer Science analyzed millions of Chinese microblogs, or "weibos," to uncover a set of politically sensitive terms that draw the attention of Chinese censors. Individual messages containing the terms were often deleted at rates that could vary based on current events or geography.
The study is the first large-scale analysis of political content censorship in social media, a topic that drew attention and controversy earlier this year when Twitter announced a country-by-country policy for removing tweets that don't comply with local laws.
In China, where online censorship is highly developed, the researchers found that oft-censored terms included well-known hot buttons, such as Falun Gong, a spiritual movement banned by the Chinese government, and human rights activists Ai Weiwei and Liu Xiaobo.
Others varied based on events; Lianghui, a term that normally refers to a joint meeting of China's parliament and its political advisory body, became subject to censorship when it emerged as a code word for "planned protest" during pro-democracy unrest that began in February 2011.
The CMU study also showed high rates of weibo censorship in certain provinces. The phenomenon was particularly notable in Tibet, a hotbed of political unrest, where up to 53 percent of locally generated microblogs were deleted.
The study by Noah Smith, associate professor in the Language Technologies Institute (LTI); David Bamman, a Ph.D. student in LTI; and Brendan O'Connor, a Ph.D. student in the Machine Learning Department, appears in the March issue of First Monday, a peer-reviewed, online journal.
"A lot of studies have focused on censorship that blocks access to Internet sites, but the practice of deleting individual messages is not yet well understood," Smith said. "
The rise of domestic Chinese microblogging sites has provided a unique opportunity to systematically study content censorship in detail.
The so-called Great Firewall of China, which prevents Chinese residents from accessing foreign websites such as Google and Facebook, is China's best known censorship tool.
Other countries also are known to block Web access, such as when Egypt shut down Twitter and other social media sites during last year's Arab Spring protests.